The 1.5" pipe for the kitchen sink runs into the basement, then wye connects to a 2" pipe (with access cap) before going under the concrete slab. The kitchen sink is vented to outside. I'm interested in putting a small sink nearby, and I was wondering if it would be acceptable to connect the new sink's 1.5" to the 2" pipe shared with the upstairs sink. As for venting, could this new sink be vented with an AAV under the cabinet? Also, I'm concerned that a full sink of water from upstairs might tend to push back up into the basement sink.
Any advice would be greatly appreciated. If this is feasible, I'm likely to hire someone for this project, but am looking to understand things before starting the process.